Television signals formatted according to all of the major broadcasts standards are interlace-scan video signals. In an interlace-scan video signal, an image frame is displayed as two interleaved fields. One field, the upper field, contains the odd lines of the image frame while the other field, the lower field, contains the even lines of the image frame. Typically, the image fields occur at different times. For example, video signals formatted according to the NTSC standard have a field interval of 1/60 of a second between successive fields.
It has been known for some time that the display of interlace-scan video images produces artifacts in the displayed images. The most noticeable of these is a vertical dot crawl in which the image picture elements (pixels) which make up vertical lines in the displayed image appear be moving toward the top of the image.
It has also been known that distortion of this type may be removed if the interlace-scan image is displayed in a non-interlace format. One method of generating a non-interlace-scan image from a interlace-scan signal is to double each line of the interlace-scan signal or to interpolate between successive lines in a field in order to order to generate these image frames, tend to have artifacts such as jagged diagonal lines and horizontal lines which oscillate up and down from field to field. The systems which interpolate the intervening lines and image fields have alternating lines of different horizontal resolution, since the horizontal resolution of the interpolated lines is one-half the resolution of the non interpolated lines. Moreover, the relative position of the high resolution and low resolution image lines changes from frame to frame causing additional artifacts.
Other interlace-scan to progressive-scan conversion systems attempt to generate additional lines in one image field from the other image field which makes up the image frame. These systems use either an adaptive filter method or a linear filter method. The adaptive methods tend to be tricked by noise in the signal. This noise in the image signal may cause the adaptive filter to make the wrong decision, which may result in visible artifacts. Systems employing conventional linear filters have the problem of different frequency responses for pixels in the one image field versus pixels in the other image field or for interpolated versus non-interpolated pixels.